Rinse asparagus and mushrooms briefly in cold water.
Trim and discard bottoms of both vegetables.
Slice asparagus diagonally into pieces 1/2 inch (1 cm) long.
Cut mushrooms, if necessary, into pieces no larger than 1 inch (2 cm).
In wok over medium heat, melt butter; add asparagus, increase heat to medium-high and stir to coat all pieces.
Cover and cook for 2 minutes, shaking wok 2 or 3 times.
Uncover wok and increase heat to high; add mushrooms, salt and pepper and stir well.
Cover and cook for 1 minute; remove lid and stir-fry for 2 to 3 minutes longer or until asparagus is tender-crisp, mushrooms are softened and moisture has evaporated.